Iron-graphite dissolving metals

The experimental procedure can be understood by reference to the Fe-C phase diagram, Figure 2. The specimen is reacted with methane at temperatures of 750-850°C. Carbon dissolves in the metal until the saturation solubility has been exceeded at the gamma-Fe/FejC phase boundary, or alternatively the specimen is cooled to a temperature below the iron-carbon eutectoid (721°C or 738°C). The solid and dotted lines in Figure 2 indicate the alternative iron-Fe3C and iron-graphite phase boundaries respectively. Either method produces similar reaction products but the isothermal experiment is prefereably carried out at higher pressures, e.g. 50 Torr, to minimise run times. 

For the production of superpurity aluminum on a large scale, the Hoopes cell is used. This cell involves three layers of material. Impure (99.35 to 99.9% aluminum) metal from conventional electrolytic cells is alloyed with 33% copper (cutcctic composition) which serves as the anode of the cell A middle, fused-salt layer consists of 60% barium chloride and 40% AlF 1.5NaF (chiolite), mp 72(TC. This layer floats above the aluminum-copper alloy. The top layer consists of superpurity aluminum (99.995%). The final product usually is cast in graphite equipment because iron and other container metals readily dissolve in aluminum. For extreme-purity aluminum, zone refining is used. This process is similar to that used for the production of semiconductor chemicals and yields a product that is 99.9996% aluminum and is available in commercial quantities. 

For anodic processes the choice of materials for the electrode is much more limited than for cathodic ones, as the anode could bo easily attacked by the products of the electrolysis (chlorine, oxygon etc.), or electrochemioally dissolved. In alkaline solutions the selection will be restricted to the application of platinum (or alloys of platinum with irridium or rhodium), palladium, carbon (or rather graphite) iron and nickel, while for acid solutions only metals of the platinum group and graphite will be suitable in a special case of the electrolysis in sulphuric acid solutions lead has found wide use, it getting coated with a conductive film of lead dioxide. 

IRON, Fe (Ar 55 85) - IRON(II) Chemically pure iron is a silver-white, tenacious, and ductile metal. It melts at 1535°C. The commercial metal is rarely pure and usually contains small quantities of carbide, silicide, phosphide, and sulphide of iron, and some graphite. These contaminants play an important role in the strength of iron structures. Iron can be magnetized. Dilute or concentrated hydrochloric acid and dilute sulphuric acid dissolve iron, when iron(II) salts and hydrogen gas are produced. 

Another important CAEM observation is that some nickel crystals deactivate at elevated temperature 1175 K and become immobile (22, 32). This phenomena was attributed to dissolution of carbon into the nickel crystallites and its eventual precipitation as graphite platelets, similar to observations on well-defined nickel (33), platinum, (34) and iron (35, 36) surfaces. Under CAEM pressure (< 1-torr H2) platelet carbon is not fully gasified and remains a hollow shell as the nickel crystal dissolves away at 1225 K. Similar phenomena may give rise to the e TPSR carbon state, which at 1-atm H2 or 20-torr H2O is gasified at higher temperature. In H2O, nickel crystallites observed in CAEM are stable and do not dissolve into the bulk at elevated temperature. However, even in the presence of 1 0, the initial step in cracking open a layer of platelet carbon or the carbon shell surrounding an immobilized Mi crystallite may be penetration by fluid metal. 

As an alternative to the use of acids, sequestering agents have been employed to dissolve the corrosion products without attacking the parent metal. The most effective formulations are based on the derivative of ethylene diamine-tetra acetic acid (EDTA). Lead artefacts from the Mary Rose were cleaned in a 10% solution of this compound. The use of EDTA is not recommended for cast iron as the graphite flakes embedded within the corrosion products are also dissolved. As with the use of acids, the shape of the artefact is altered if the corrosion layers are very thick and it is also difficult to wash out all the solutions from cracks, crevices and pores in the artefact after cleaning. 

The metal is produced on a massive scale by the Hall-Heroult method in which aluminum oxide, a nonelectrolyte, is dissolved in molten cryolite and electrolyzed in a large cell. The bauxite contains iron oxide and other impurities, which would contaminate the product, so the bauxite is dissolved in hot alkali, the impurities are removed by filtration, and the pure aluminum oxide then precipitated by acidification. In the cell, molten aluminum is tapped off from the base and carbon dioxide evolved at the graphite anodes, which are consumed in the process. The aluminum atom is much bigger than boron (the first member of group 13) and its ionization potential is not particularly high. Consequently aluminum forms positive AT ions. However, aluminum also has nonmetallic chemical properties. Thus, it is amphoteric and also forms a number of covalently bonded compounds. 

Metallic copper is sometimes obtained by leaching a copper ore with sulfuric acid and then depositing the metal by electrolysis of the copper sulfate solution. Most copper ores, however, are converted into crude copper by chemical reduction. This crude copper is cast into anode plates about 2 cm thick, and is then refined electrolytically. In this process the anodes of crude copper alternate with cathodes of thin sheets of pure copper coated with graphite, which makes it possible to strip off the deposit. The electrolyte is copper sulfate. As the current passes through, the crude copper dissolves from the anodes and a purer copper deposits on the cathodes. Metals below copper in the EMF series, such as gold, silver, and platinum, remain undissolved, and fall to the bottom of the tank as sludge, from which they can be recovered. More active metals, such as iron, remain in the solution. 

The properties of diamond, cubic boron nitride, and conventional hard materials are summarized in Table 9.1. In addition to being the hardest known substance, diamond is chemically inert to essentially aU environments below a temperature of about 500°C and is therefore uniquely qualified for many applications. Diamond has a cubic structure, with each carbon atom bonded to four nearest neighbors. Cleavage normally occurs on one of four (111) planes. In addition to its intrinsic brittleness, diamond has two important limitations. Diamond begins to oxidize and/or graphitize rapidly at temperatures above 600-700°C in air or an oxidizing atmosphere. Diamond readily dissolves in and can be graphitized by ferrous metals such as iron, steels, nickel, and nickel-based superaUoys, and therefore abrasion resistance with these metals is poor. 

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